Mozilla Firefox before 48.0 and Firefox ESR 45.x before 45.3 preserve the network connection used for favicon resource retrieval after the associated browser window is closed, which makes it easier for remote web servers to track users by observing network traffic from multiple IP addresses.

It was reported that once the favicon is requested from a site, the remote server can keep the favicon network connection open even when the page is later closed. This allows a malicious site to continue to use this channel to send requests to the browser, leading to potential information disclosure, such as tracking the user across multiple IP addresses as the user changes networks.